This invention relates to a method and apparatus for igniting a High Intensity Discharge (HID) lamp. The invention has particular application in high volume commercial HID devices, where cost is an important consideration.
HID lamps typically use a gas sealed within a glass container which conducts electricity and emits light at a particular wavelength. The wavelength is a function of the type of gas used.
In order to start, or ignite, an HID lamp, there are generally four phases the ballast must account for. The first phase is the breakover phase, in which a relatively high voltage pulse (e.g., 3 kilovolts) is applied between two electrodes of the HID lamp in order to free electrons from the gas molecules and start the conduction process. Typically the ballast must supply the high voltage pulse for a duration of approximately 10 microseconds. After the 10 microsecond 3 kilovolt pulse, a takeover state is entered. Depending on the lamp and ballast conditions, the takeover state can last on the order of hundreds of microseconds, during which the ballast must be capable of supplying approximately 280 to 300 volts to the lamp. This continues the process of bringing the gas towards a steady state of conduction.
After takeover, the HID lamp enters the run up phase. At the beginning of the run up phase, the temperature, internal pressure, and voltage within the lamp are relatively low. During the run up phase, the voltage ramps up from approximately 20 volts to approximately 90 volts over the course of a minute or even more. After that minute, the lamp enters its fourth and final stage, which is the steady state operating phase. During steady state, the lamp emits light at its normal temperature and pressure for which it was designed.
During steady state, the lamp is operating based upon a current signal which must oscillate. More specifically, because of the physics of such devices, they cannot operate on DC but must instead be operated based upon preferably a low frequency square wave signal which oscillates between a positive and negative current. Thus, the steady state may be, for example, a square wave current at 100 Hz that results in a lamp voltage which oscillates between plus and minus 90 volts.
The above four phases require that the circuitry to drive the lamp deliver a prescribed signal. More specifically, the drive circuitry must deliver the breakover ignition pulse of approximately 10 microseconds, followed by the takeover voltage of approximately 280 to 300 volts for on the order of hundreds of microseconds, and then the run up and steady state voltages. FIG. 1 is an exemplary prior art arrangement for delivering the above-prescribed signal. At ignition, a signal of approximately 400 volts is placed across capacitor 150. The 400 volts is conveyed through device 130 and inductor 134, and causes a signal of approximately 300 volts to appear across capacitor 132.
Capacitor 132 causes igniter 105 to generate a pulse of approximately 3 kilovolts for approximately 10 microseconds, after which the igniter 105 appears essentially as a short circuit. The igniter is typically triggered by the voltage across capacitor 132 to generate the 3 kilovolt pulse. After the initial pulse, and when the igniter acts as an effective short circuit, the voltage of approximately 280 to 300 volts from capacitor 132 is delivered from capacitor 132 through igniter 105 to the HID lamp 108. These 280-300 volts are maintained for on the order of hundreds of microseconds, until the takeover phase is complete. Immediately after the takeover phase, controller 110 begins the run up and steady state process. During steady state, controller 110 controls the gate voltages of 136 through 139 such that the oscillating square wave described above is delivered to the lamp 108.
A problem with the arrangement of FIG. 1 is that the cost is relatively expensive due to the number of components. More specifically, because it is required to generate a square wave which varies its polarity periodically, four transistors are required within commutator 120. The four transistors act in conjunction with the control voltages applied to their gates by controller 110 in order to generate the required square wave.
FIG. 2 shows an alternative prior art embodiment for delivering the prescribed four phases of signal to an HID lamp. The arrangement of FIG. 2 utilizes two capacitors 220 and 222 in series as a voltage divider. HID lamp 108 is connected between igniter 105 and point 208. The system need not use four different transistors to create the square wave utilized during steady state. Instead, only two transistors 224 and 226 are needed. During steady state, lamp electrode 210 is connected to point 208, and transistors 224 and 226 can be operated at high frequency and at varying duty ratios. Thus, by operating controller 218 in a fashion such that transistors 224 and 226 are alternatively switched on and off with the proper durations, the required steady state current and voltage waveforms can be delivered. Since one of the HID lamp electrodes is connected to the middle of the divider formed by capacitors 220 and 222, only two transistors 224 and 226 are needed to generate a square wave with changing polarity.
The problem with the arrangement of FIG. 2 occurs during ignition. More specifically, in order to supply the approximately 280 volts needed to be present across the lamp 108 during takeover phase, 560 volts must be present between points 214 and 216. This increased voltage, which is used only during the initial ignition process, creates relatively high stress on the circuit components. This either causes failures or, in the case of quality components that can withstand the stress, drives up the cost to nearly the point of using the four devices in FIG. 1.
In view of the above, there exists a need in the art for an ignition circuit for an HID lamp which can utilize only a small number of switching devices but yet can operate without the relatively high voltages required in arrangements such as that in FIG. 2.
The above and other problems of the prior art are overcome in accordance with the teachings of the present invention which relates to an ignition circuit for HID lamps. A two-transistor circuit is utilized which is sufficient to provide the steady state square wave voltage of approximately 90 volts. During the ignition process, a switch is utilized to switch one of two capacitors forming a voltage divider out of the ignition circuit. This results in the entire input voltage being applied to one capacitor, thereby delivering a sufficient voltage for ignition. After the ignition period, the second capacitor is switched back into the circuit, thereby forming a voltage divider and permitting the pulsed steady state voltage to be delivered to the lamp.